It is known that the treatment of sludge originating from water purification stations is commonly carried out by dehydration on a belt filter, this dehydration being preceded by a thickening operation which is carried out in a storage vessel which may be either static or gently agitated.
The present titleholder has in recent years developed a draining grid which must be washed countercurrentwise at regular intervals, this draining grid having been the subject of French Patent 85/07,026, which was applied for on May 9, 1985 and published under number 2,564,637. This draining grid, which replaces the thickener, makes it possible to produce less bulky dehydration plants which are easier to deodorize and, as a result, are less costly.
To limit the size of the dehydration plants, the draining grid and the belt filter have had to be placed side by side to form single machines, the diluted sludge entering at the grid side and leaving again after being pressed between the belts of the filter, into the form of dehydrated "cakes". However, such a device is limited in compactness, since the draining grid requires washing by countercurrent (downward) spraying with water. In such a known plant it is therefore impossible to arrange the grid part above the pressing-belt part of the filter, since the height needed for placing the countercurrent washing system under the grid necessitates an excessive height between this grid and the pressing belt, and this results in the destruction of the sludge which is flocculated with polymers when this sludge falls from the grid to the belt.